For the storage of heat in a storage material suitable for this purpose, the temperature of said storage material is normally increased. This form of heat storage is called sensible heat storage.
Whenever, in a material suitable for this purpose, a phase transition takes place, for example the transition from the solid into the liquid phase (or vice versa), the relationship between the temperature of the storage material, and the heat absorbed (or output) by the storage material is no longer linear, in the event of a transition from solid to liquid, the heat storage material begins to melt when the temperature of the phase transition is reached. The storage material maintains this temperature until the storage material has completely melted. Only then does an increase in the temperature occur again upon further absorption of heat.
Since virtually no increase in the temperature occurs for a relatively long time despite the supply of heat, this is called latent heat. In the case, for example, of the typical phase transition solid/liquid, the latent heat is identical to the melting heat or crystallization heat of the storage material.
A latent heat storage material has the great advantage that it can be used to store relatively large quantities of heat within a small temperature range. Since a phase transition takes place over a certain period of time at a substantially constant temperature, temperature fluctuations can be compensated for and temperature peaks avoided.
Latent heat storage materials are known in various forms. In English terminology, these materials are also called PCM materials (phase change materials).
At a target temperature (temperature of the phase transition) of approximately 0° C., water with different additives can be used as the latent heat storage material. For cold storage below 0° C., use is made, for example, of suitably prepared salt solutions.
In the region just above 0° C., other materials, for example those on the basis of paraffins, are more suitable.
In particular, as background, reference is made to the overview article from BINE Information Service “Themeninfo IV/02 aus dem Jahre 2002 [Theme info IV/02 from 2002]”, (FIZ Karlsruhe, project code 0329840A-D, which is retrievable at www.bine.info, keyword: “Latentwärmespeicher [Latent heat storage]”). For the general background of latent heat storage materials and the use possibilities thereof, reference is hereby made by reference to the content of this citation.
A latent heat storage element according to the present invention is a latent heat storage material in a closed covering which is optionally also provided with a pressure-equalizing valve. A macro-encapsulated PCM material is also mentioned in this respect. The covering is frequently made from plastic. The basic structure is known, for example, from what are referred to as cooling batteries.
Latent heat storage elements of this type can be considered individually or else as a plurality of latent heat storage elements installed, for example, in a corresponding container.
There are, in the meantime, latent heat storage elements of the type under discussion for an abundance of target temperatures, in particular also from, the applicant (brochure “va-Q-tec Packaging Portfolio, January 2011”). Latent heat storage elements for target temperatures of 37° C., 22° C., 4° C., 0° C., −19° C., −21° C. and −32° C. are found, there. Other providers have comparable latent heat storage elements in their sales range, sometimes also for different target temperatures.
Latent heat storage elements of the type under discussion are used in a particular field of use in thermally insulated containers, in particular for transport purposes. For example, this is true of the transport of temperature-sensitive items, such as pharmaceuticals, biotechnological products, test apparatuses or samples for and from clinical studies, transplant items or blood reserves. In this field of use, the optimum transport and storage temperature which absolutely has to be maintained is, for example, 2° C. to 3° C. The products are frequently only stable at ail within a very narrow temperature range. The products therefore absolutely have to be transported and stored within this temperature range. In addition, frequently such products, which are highly sensitive with regard to the transport temperature, must not ever freeze. Temperatures below 0° C. most then be reliably avoided. The target temperature thus has to be reliably reached and maintained with, comparatively little deviation.
The temperature which is maintained with little deviation by the latent heat storage element during the phase transition and which results from the used latent heat storage material of the latent heat storage element is designated below as the target temperature.
The prior art on which the present invention is based (WO 2004/104498 A2) relates to a thermally insulated container, in particular for transport purposes, wherein the container Is insulated against heat exchange with the ambient atmosphere, preferably by vacuum insulation panels, and has at least one latent heat storage element of the type under discussion in the interior. To this extent, reference is made to the disclosure of the previously published document from which many aspects of the use of latent heat storage elements in transport containers can be gathered.
The technology of vacuum insulation panels has basically already been known for a relatively long time, but is being continuously perfected in terms of production and material. For vacuum insulation panels, reference may be made in principle to this extent to DE 100 58 566 C2 belonging to the applicant of the present application. Vacuum insulation panels of this type are currently the most efficient thermal insulation elements.